I. Field of the Invention
The present invention relates generally to a femoral hip prosthesis and, more particularly, to a femoral component which can be stress tailored to optimally share the load with the femur in which it is implanted.
II. Description of the Prior Art
Based on the precepts of Wolff's Law which states that bone tissue will remodel in direct relation to the stress applied to it, it is desirable to stress bone at an optimal level to minimize and control remodeling after THR (total hip replacement) arthroplasty. Usually some degree of proximal femur bone remodeling accompanies total hip replacement. Due to mechanical stiffness, metallic implants typically stress protect the proximal bone to some extent. In patients with relatively large intramedullary canals which require a large diameter implant for optimal fit, stress protection may be particularly troublesome. In the most extreme case, the proximal femoral bone may resorb to a small fraction of its original mass, possibly causing a loss of support of the implant or implant breakage. It is unfortunate that implant flexural stiffness increases at an exponential rate, typically at powers between two and four, depending upon implant geometry, relative to linear increases in implant dimension. Further aggravating the situation is the fact that there is little correlation between the size of the patient and the diameter of the intramedullary canal. That is, a small, relatively light person may have a femur with a large diameter canal and a much larger person may have a femur with a smaller diameter canal. Therefore, it is desirable to produce an implant, especially a larger diameter implant, with greatly reduced stiffness in relation to its mass.
This can be accomplished in several ways. For example the use of materials which are inherently less stiff, that is, possess a lower flexural modulus might be considered. Thus, the use of titanium alloy or a carbon fiber reinforced polymer composite in lieu of the stiffer cobalt-chrome alloy might be considered. An implant can also be hollowed out. This method is marginally effective, however, due to the fact that the centrally located material contributes little to the stiffness of the implant. For example, if an implant with a round stem of 16 mm diameter is hollowed to a wall thickness of only 2 mm, the resulting decrease in flexural stiffness is only 32% while the decrease in mass is 56%. Interestingly, a 16 mm diameter stem is 6.5 times stiffer than the 10 mm diameter stem. Morscher and Dick reported on nine years of clinical experience with a so-called "isoelastic" shaft prosthesis manufactured using polyacetal resin to transmit forces from the pelvis through the femoral head and neck into the femur in their paper: "Cementless Fixation of `Isoelastic` Hip Endoprostheses Manufactured from Plastic Materials", Clinical Orthopaedics, June, 1983, Volume 176, pages 77-87. They stated: "The optimal fixation of an implant depends mainly on its design and material. The insertion of an artificial joint induces remodeling of the bony structures. If stability is not achieved, the implant sooner or later will loosen. The elasticity, and consequently the deformation, of an implant depend on the elastic modulus of the material and on the prosthetic design. By adjusting the physical characteristics of the foreign material to that of the bone tissue, as well as the design of the prosthesis to the femoral shaft, the entire system would have the same elasticity as a normal femur. A more elastic hip endoprosthesis also may act as a shock absorber during walking, particularly in the heel/strike and toe/off phases."
They proceeded to explain that this was the concept of the "isoelastic" hip endoprosthesis manufactured by Robert Mathys and implanted in 1973. In this instance, the prosthesis was composed of polyacetal resin which has an elasticity modulus approaching that of bone tissue, good durability, and tenacity for highly stressed components in combination with good tissue tolerance. To achieve the acquired structural strength in the neck portion, the component was reinforced by a metallic core that was tapered toward the tip to increase the elasticity of the stem, thereby allowing the stem of the prosthesis to follow the deformation of the bone. In commenting on the design, the authors further stated: "Isoelasticity implies the optimum approximation of the physical characteristics of an implant to those of the bone. An ideal isoelasticity, however, can never be achieved, since bone is anisotropic and the alloplastic materials used for joint arthroplasty show isotropic properties. In addition, there is no adaptation of the structures to the forces acting on the hip, as in the case in viable bone. Moreover, the variety of individual forms and strengths of human bone can never be imitated by an artificial joint. Use of more elastic materials, however, should avoid the disadvantages of the rigid materials used to date."
U.S. Pat. No. 4,287,617 to Tornier discloses a hip prosthesis with a femoral stem which provides a measure of the elasticity spoken of by Morscher and Dick. A transverse section of the Tornier stem is in the form of a substantially rectangular tube of which one of the small sides is virtually cut away so as to leave a very large slot. The C-shaped section thus obtained is said to exhibit excellent transverse elasticity which facilitates the positioning of the pin in the medullary cavity by insertion. Other stated advantages are that the pin is not as heavy as solid designs, and that the cavity encourages bone growth.
An alternate approach to the foregoing is the subject of commonly assigned U.S. application Ser. No. 151,627 to co-inventor Todd S. Smith entitled "Controlled Stiffness Femoral Hip Implant". In that construction, the medial side of the length of the implant is milled out to form a channel shaped stem cross section. The amount of material removed determines the resulting decrease in stiffness of the implant while the outside geometry remains substantially unchanged with the exception of the open channel on the medial side of the implant. The resulting longitudinal channel lies generally in the coronal plane when the stem is in the implanted condition. The depth of the channel is variable between the proximal and distal ends of the femoral implant so as to affect the mass moment of inertia at any given location along a length of the stem to thereby achieve an optimal stem flexibility. That is, the stem is so formed that at specified locations along its length, it substantially correlates to the flexibility of the femur itself.